1. Field of the Invention
The present invention relates to a semiconductor device including an optical device and an electron device and a method for producing the same. In particular, the present invention relates to an optical device such as an integrable wavelength-controlled semiconductor laser and also to a high speed and low power consumption electron device.
2. Description of the Prior Art
Recently, a distributed Bragg reflection (DBR) laser and a distributed feedback (DFB) laser have been developed. These types of lasers have excellent characteristics as a single longitudinal mode laser.
The wavelength of light reflected in parallel generated from parallel incident light in a diffraction grating is expressed by the formula: .lambda..sub.0 =2N.sub.eff .LAMBDA./m, where N.sub.eff is an equivalent refractive index, .LAMBDA. is the periodicity of the diffraction grating, and m is the order of the diffraction grating. In the DBR laser and the DFB laser, oscillation in a single longitudinal mode is realized by utilizing wavelength selectivity.
FIG. 9 is a schematic view of an AlGaAs DFB laser as an example of such known semiconductor lasers. The DFB laser shown in FIG. 9 is produced in the following manner. On a GaAs substrate 901, a cladding layer 902, an active layer 903, and an optical guide layer 904 are sequentially grown by a first epitaxial growth, and a diffraction grating 907 having a specific periodicity is formed on a surface of the optical guide layer 904. Then, a cladding layer 905 and a contact layer 906 are grown by a second epitaxial growth on the diffraction grating 907.
As is apparent from the above, in order to produce a semiconductor laser such as the one shown in FIG. 9, the epitaxial growth must be performed at least twice. Moreover, it is necessary to form a diffraction grating at a sufficiently close position to an active layer so that the diffraction grating can fully influence an optical field intensity distribution which is generated in the optical waveguide region including the active layer. In a semiconductor laser having such a structure, a non-radiative recombination center which is generated at the interface along the diffraction grating reduces the light emitting efficiency and the reliability of the laser. Moreover, freedom of design of device structure is restricted.
FIG. 10 is a schematic view of an InP DFB laser as another example of known semiconductor lasers. In the InP DFB laser, as distinct from the AlGaAs DFB laser, no oscillating light is absorbed into the substrate. Therefore, it is possible to form a diffraction grating 1006 on a substrate 1001. Accordingly, the rest of the multiple layer structure including an optical guide layer 1002, an active layer 1003, a cladding layer 1004, and a contact layer 1005 can be formed by performing an epitaxial growth only once. However, as in the AlGaAs DFB laser shown in FIG. 9, it is required that the active layer 1003 be positioned sufficiently close to the diffraction grating 1006. Accordingly, the semiconductor laser of FIG. 10 also has the problems that the light emitting efficiency and the product reliability are reduced and freedom of design of the device structure is restricted.
An optical device and an electron device employing an ultra thin semiconductor film have recently been studied in order to utilize a variety of quantum effects for obtaining higher performance.
FIG. 11 is a schematic view of a quantum well laser as an example of such devices. In the quantum well laser, the movement of electrons and holes is confined within a quantum well layer 1102 in a thickness direction by a pair of confining layers 1101 and 1103. Therefore, the quantum well laser has advantages such as lowered threshold current density and improved temperature characteristics, as described in Electronics Letters, vol. 18, 1095 (1982).
In such a quantum well, however, the electrons and holes are quantized only in a thickness direction, but not in a direction on a plane parallel to a main surface of the quantum well layer. Therefore, the reduction of the threshold current density and the improvement of the temperature characteristics are not sufficient. In order to solve these problems, semiconductor lasers having curved active layers as shown in FIGS. 12 and 13 are proposed. The lasers of FIGS. 12 and 13 can attain quantum wire effects and quantum box effects, respectively (Japanese Laid-Open Patent Publication Nos. 61-212084 and 61-212085, Journal of Crystal Growth 104 (1990), pp. 766-772).
In each of the lasers of FIGS. 12 and 13, a single active layer or a multiple active layer structure is formed as a gain area. The active layer is sufficiently thin to generate quantum effects and includes a diffraction grating having a sufficiently short periodicity to generate additional quantum effects in a direction on a plane parallel to a main surface of the active layer. By this construction, quantum wire effects and quantum box effects are obtained, namely, the movement of the electrons and holes is confined in a direction of the periodicity of the corrugation as well as in the thickness direction. As a result, semiconductor devices with improved performance are produced.
However, in order to form an active layer having a diffraction grating which is appropriate to obtain quantum effects, it is necessary to position the active layer within a short distance from the corresponding corrugation formed on the substrate. This means that the freedom in designing the device structure is restricted. Moreover, it is difficult to avoid the problems arising from the non-radiative recombination center generated at the interface along the corrugation formed on the substrate.